tracing: document TRACE_EVENT macro in tracepoint.h

Impact: clean up / comments

Kosaki Motohiro asked about an explanation to the TRACE_EVENT macro.
Ingo Molnar replied with a nice description.

This patch takes the description that Ingo wrote (with some slight
modifications) and adds it to the tracepoint.h file.

Reported-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index c7b09452514b9267e97bd4ade5e53493932ea817..119ece224c21611aace1dcd1a06d6a73c7ddea61 100644 (file)
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -157,6 +157,109 @@ static inline void tracepoint_synchronize_unregister(void)
 #define TRACE_FORMAT(name, proto, args, fmt)           \
        DECLARE_TRACE(name, PARAMS(proto), PARAMS(args))
 
+
+/*
+ * For use with the TRACE_EVENT macro:
+ *
+ * We define a tracepoint, its arguments, its printk format
+ * and its 'fast binay record' layout.
+ *
+ * Firstly, name your tracepoint via TRACE_EVENT(name : the
+ * 'subsystem_event' notation is fine.
+ *
+ * Think about this whole construct as the
+ * 'trace_sched_switch() function' from now on.
+ *
+ *
+ *  TRACE_EVENT(sched_switch,
+ *
+ *     *
+ *     * A function has a regular function arguments
+ *     * prototype, declare it via TP_PROTO():
+ *     *
+ *
+ *     TP_PROTO(struct rq *rq, struct task_struct *prev,
+ *              struct task_struct *next),
+ *
+ *     *
+ *     * Define the call signature of the 'function'.
+ *     * (Design sidenote: we use this instead of a
+ *     *  TP_PROTO1/TP_PROTO2/TP_PROTO3 ugliness.)
+ *     *
+ *
+ *     TP_ARGS(rq, prev, next),
+ *
+ *     *
+ *     * Fast binary tracing: define the trace record via
+ *     * TP_STRUCT__entry(). You can think about it like a
+ *     * regular C structure local variable definition.
+ *     *
+ *     * This is how the trace record is structured and will
+ *     * be saved into the ring buffer. These are the fields
+ *     * that will be exposed to user-space in
+ *     * /debug/tracing/events/<*>/format.
+ *     *
+ *     * The declared 'local variable' is called '__entry'
+ *     *
+ *     * __field(pid_t, prev_prid) is equivalent to a standard declariton:
+ *     *
+ *     *       pid_t   prev_pid;
+ *     *
+ *     * __array(char, prev_comm, TASK_COMM_LEN) is equivalent to:
+ *     *
+ *     *       char    prev_comm[TASK_COMM_LEN];
+ *     *
+ *
+ *     TP_STRUCT__entry(
+ *             __array(        char,   prev_comm,      TASK_COMM_LEN   )
+ *             __field(        pid_t,  prev_pid                        )
+ *             __field(        int,    prev_prio                       )
+ *             __array(        char,   next_comm,      TASK_COMM_LEN   )
+ *             __field(        pid_t,  next_pid                        )
+ *             __field(        int,    next_prio                       )
+ *     ),
+ *
+ *     *
+ *     * Assign the entry into the trace record, by embedding
+ *     * a full C statement block into TP_fast_assign(). You
+ *     * can refer to the trace record as '__entry' -
+ *     * otherwise you can put arbitrary C code in here.
+ *     *
+ *     * Note: this C code will execute every time a trace event
+ *     * happens, on an active tracepoint.
+ *     *
+ *
+ *     TP_fast_assign(
+ *             memcpy(__entry->next_comm, next->comm, TASK_COMM_LEN);
+ *             __entry->prev_pid       = prev->pid;
+ *             __entry->prev_prio      = prev->prio;
+ *             memcpy(__entry->prev_comm, prev->comm, TASK_COMM_LEN);
+ *             __entry->next_pid       = next->pid;
+ *             __entry->next_prio      = next->prio;
+ *     )
+ *
+ *     *
+ *     * Formatted output of a trace record via TP_printk().
+ *     * This is how the tracepoint will appear under ftrace
+ *     * plugins that make use of this tracepoint.
+ *     *
+ *     * (raw-binary tracing wont actually perform this step.)
+ *     *
+ *
+ *     TP_printk("task %s:%d [%d] ==> %s:%d [%d]",
+ *             __entry->prev_comm, __entry->prev_pid, __entry->prev_prio,
+ *             __entry->next_comm, __entry->next_pid, __entry->next_prio),
+ *
+ * );
+ *
+ * This macro construct is thus used for the regular printk format
+ * tracing setup, it is used to construct a function pointer based
+ * tracepoint callback (this is used by programmatic plugins and
+ * can also by used by generic instrumentation like SystemTap), and
+ * it is also used to expose a structured trace record in
+ * /debug/tracing/events/.
+ */
+
 #define TRACE_EVENT(name, proto, args, struct, assign, print)  \
        DECLARE_TRACE(name, PARAMS(proto), PARAMS(args))